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Variable Stars and the Asymptotic Giant Branch: Stellar Pulsations, Dust Production, and Mass Loss

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Variable Stars and the Asymptotic Giant Branch: Stellar Pulsations, Dust Production, and Mass Loss 244 Speck, JAAVSO Volume 40, 2012 Variable Stars and the Asymptotic Giant Branch: Stellar Pulsations, Dust Production, and Mass Loss Angela K. Speck Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211; [email protected] Presented at the 100th Spring Meeting of the AAVSO, May 23, 2011; received February 17, 2012; revised March 26, 2012; accepted May 11, 2012 Abstract Low- and intermediate-mass stars (1–8 MÄ; LIMS) are very important contributors of material to the interstellar medium (ISM), and yet the mechanisms by which this matter is expelled remain a mystery. In this paper we discuss how interferometry plays a role in studying the interplay between pulsation, mass loss, dust formation and evolution of these LIMS. 1. Introduction 1.1. The importance of cosmic dust At the beginning of the Universe, all matter was in the form of hydrogen and helium: all elements heavier than helium form via nuclear fusion in stars. Newly-formed elements are ejected from stars either explosively (in the case of supernovae) or more gently over a few hundred thousand years for lower- mass stars like the Sun. These new elements then become part of the interstellar medium (ISM), from which new stars and their planets form. With the emergence of infrared (IR) astronomy in the late 1960s, the importance of dust particles in the Universe began to be revealed. Dust is a vital ingredient in many astrophysical environments (Videen and Kocifaj 2002; Draine 2003; Krishna Swamy 2005). It plays an essential role in star formation processes, and contributes to several aspects of interstellar processes such as gas heating and molecule formation (Krügel 2008). In addition, since mass loss from evolved stars is driven by radiation pressure on dust grains, it is intimately linked to the precise nature of the circumstellar dust (Woitke 2006). Furthermore, dust has been observed at higher redshifts than expected, and understanding this phenomenon is vital to our understanding of the cosmos at large and its evolution (Sloan et al. 2009; Bussmann et al. 2009). Moreover, the detection of dust at high redshift raises concerns about the use of standard candles (for example, Type Ia Supernovae) as accurate distance indicators (Jain and Ralston 2006). Understanding the dust at high redshift is vital to cosmological models and dark energy studies (Corasaniti 2006; Jain and Ralston 2006). Dust needs to be well understood in its own right, if we are to understand how it contributes to many aspects of astrophysics. Speck, JAAVSO Volume 40, 2012 245 1.2. Low- and intermediate-mass stars (LIMS) The type of stars that produce the majority of the dust complement for the Galaxy start their lives as low- and intermediate-mass stars (0.8–8MÄ; LIMS). Up to 95% of stars are LIMS (Kwok 2004). Studying the nature of dust around LIMS is important for three reasons: (1) this is where the dust originates, and thus knowing its initial state will allow us to predict more accurately its fate in and effect on the ISM and beyond; (2) the environment around most of these LIMS is relatively benign (little UV) and thus has simplified chemistry, which aids in our attempts to understand the processes in play and test current hypotheses of dust formation (which are also applied to many, more complex astrophysical environments); and (3) the evolution of LIMS is intimately linked to their dust production, and thus a feedback loop exists between dust production and stellar evolution. The precise nature of the dust grains must be assessed in order to understand this evolution. Since LIMS are major contributors of new elements to the ISM from which the next generation of stars and planets form, understanding their contribution to the ISM is crucial to our understanding of Galactic and Universal chemical evolution. In fact, mass loss is the main reason that LIMS do not explode as supernovae. We cannot understand mass loss fully until we understand the physical nature of the dust. As will be discussed below, interferometric techniques can provide data on evolved LIMS that are essential to understanding dust formation. 2. Stellar evolution LIMS eventually evolve off the main-sequence to the red giant branch and subsequently become asymptotic giant branch (AGB) stars, ending their lives as cooling white dwarfs. Between the AGB phase and the white dwarf phase some of these stars may become planetary nebulae (PNe) as the previous AGB mass loss is illuminated by the shrinking, heating central core. However, precisely which AGB stars go through the PNe phase is not clear (see, for example, Sahai et al. 2010, and references therein). 2.1. Asymptotic giant branch stars As LIMS evolve they become asymptotic giant branch (Iben and Renzini 4 1983) stars: luminous (L´ ≈ 10 LÄ), cool (Teff ≈ 3000 K) giants (R´ ≈ 1 AU), which –7 –4 lose mass at high rates (10 to a few times 10 MÄ/yr). AGB stars pulsate due to dynamical instabilities, leading to intensive mass loss and the formation of a circumstellar shell of gas. Pulsations levitate atmospheric material, allowing it to achieve an altitude where temperatures permit molecules to form, followed by the formation of small particles (dust grains). The dust grains tap into the tremendous luminosity power of the star and drive a radiation-pressured wind (see, for example, Höfner and Dorfi 1997), leading to a circumstellar outflow of dust and gas. This outflow (wind) causes AGB stars to lose mass at such 246 Speck, JAAVSO Volume 40, 2012 tremendous rates that they wither into white dwarfs rather than explode as supernovae. Generally, the mass-loss rate, M ∙ , increases over time as an AGB star evolves, and ends in an episode of extremely high mass loss, the superwind (SW) phase (Iben and Renzini 1983; Bowen 1988; Bowen and Willson 1991; Blöcker and Schönberner 1991; Vassiliadis and Wood 1993; Willson 2000). ∙ –5 –1 During the SW phase M exceeds 10 MÄ/yr . Continued AGB star mass loss causes the dust shell to increase in depth both optically and geometrically as mass-loss rate increases, shown schematically in Figure 1. As these stars approach the SW phase they become invisible at optical wavelengths and very IR-bright. During this SW stage, intense mass loss depletes the remaining hydrogen in the star’s outer envelope, and terminates the AGB phase. The rapid depletion of material from the outer envelope of the star means that while AGB mass loss may last for > 105 yrs, this extremely high mass-loss SW phase must have a relatively short duration (a few × 104 years; Volk et al. 2000). During their ascent of the AGB, these stars also evolve chemically, starting with oxygen-rich atmospheres. Helium burning forms 12C, which is dredged up to the stellar surface by strong convection currents in the mantle. Thus, carbon is injected into the stellar atmosphere. The stability of the CO molecule in the stellar atmosphere means that the carbon-to-oxygen ratio (C/O) controls the chemistry around the star: whichever element is less abundant will be entirely locked into CO molecules, leaving the more abundant element to control dust formation. Therefore, AGB stars can be either oxygen-rich or carbon-rich. For the O-rich AGB stars C/O can vary from approximately cosmic C/O ≈ 0.4) to just less than unity. Once C/O is greater than unity these stars become C-rich. Other nuclear processes (for example, the s-process) also occur in the He- and H-burning shells of AGB stars and thus other new elements are also dredged up and enrich the dust formation region. For a more detailed description of AGB stars we refer to Habing (1996) and Habing and Olofsson (2004). 2.2. Post-AGB stars Once the AGB star has exhausted its outer envelope, the AGB phase ends. At this stage the mass loss virtually stops, and the circumstellar gas and dust shell begin to drift away from the star. At the same time, the central star begins to shrink and heat up from ~ 3000 K until it is hot enough to ionize the surrounding gas, at which point the object becomes a planetary nebula (PN). The short-lived post-AGB phase, as the star evolves toward to the PN phase, is also known as the proto- or pre-planetary nebula (PPN) phase. However, not all post-AGB stars will become PNe; for some post-AGB objects the expansion speed of the circumstellar shell, combined with its density, will preclude a visible nebula of ionized gas. (Indeed, the term pre-PN was adopted to replace proto-PN to reflect the idea that not all PPNe will end up as PNe.) As the detached dust shell drifts away from the central star, the dust cools, causing a PPN to have cool IR colors. Meanwhile, the dust shell spreads out, Speck, JAAVSO Volume 40, 2012 247 becoming less dense and optically thinner, leading to changes in its spectral characteristics that may also be related to an evolution in the intrinsic nature of the dust grains (that is, composition, crystal structure, grain size, and grain shape, not just optical depth and temperature). This structural evolution of the dust shell is illustrated schematically in the upper panel of Figure 1. This post- AGB evolution of the circumstellar envelope changes its appearance, revealing features that were hidden during the AGB phase. The geometry of the dust shell also changes. Whereas observations suggest that the AGB phase has mostly spherically-symmetric mass loss, there is clearly a deviation from spherical symmetry somewhere in the evolution of these stars and their mass loss, since PNe are rarely spherical. It has been suggested that mass loss can explain the structural changes alone (Dijkstra and Speck 2006).
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